16,532 research outputs found
Cylindrical wormholes with positive cosmological constant
We construct cylindrical, traversable wormholes with finite radii by taking
into account the cut-and-paste procedure for the case of cosmic string
manifolds with a positive cosmological constant. Under reasonable assumptions
about the equation of state of the matter located at the shell, we find that
the wormhole throat undergoes a monotonous evolution provided it moves at a
constant velocity. In order to explore the dynamical nonlinear behaviour of the
wormhole throat, we consider that the matter at the shell is supported by
anisotropic Chaplygin gas, anti-Chaplygin gas, or a mixed of Chaplygin and
anti-Chaplygin gases implying that wormholes could suffer an accelerated
expansion or contraction but the oscillatory behavior seems to be forbidden.Comment: 5 pages, no figures. Accepted for publication in PRD.
(http://link.aps.org/doi/10.1103/PhysRevD.88.027507
Concise Review: The Potential Use of Intestinal Stem Cells to Treat Patients With Intestinal Failure.
: Intestinal failure is a rare life-threatening condition that results in the inability to maintain normal growth and hydration status by enteral nutrition alone. Although parenteral nutrition and whole organ allogeneic transplantation have improved the survival of these patients, current therapies are associated with a high risk for morbidity and mortality. Development of methods to propagate adult human intestinal stem cells (ISCs) and pluripotent stem cells raises the possibility of using stem cell-based therapy for patients with monogenic and polygenic forms of intestinal failure. Organoids have demonstrated the capacity to proliferate indefinitely and differentiate into the various cellular lineages of the gut. Genome-editing techniques, including the overexpression of the corrected form of the defective gene, or the use of CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 to selectively correct the monogenic disease-causing variant within the stem cell, make autologous ISC transplantation a feasible approach. However, numerous techniques still need to be further optimized, including more robust ex vivo ISC expansion, native ISC ablation, and engraftment protocols. Large-animal models can to be used to develop such techniques and protocols and to establish the safety of autologous ISC transplantation because outcomes in such models can be extrapolated more readily to humans.The field of intestinal stem cell biology has exploded over the past 5 years with discoveries related to in vivo and in vitro stem cell identity and function. The goal of this review article is to highlight the potential use of these cells to treat various epithelial disorders of the gut and discuss the various roadblocks that will be encountered in the coming years
Black holes in Einstein-Gauss-Bonnet gravity with a string cloud background
We obtain a black hole solution in the Einstein-Gauss-Bonnet theory for the
string cloud model in a five dimensional spacetime. We analyze the event
horizons and naked singularities. Later, we compute the Hawking temperature
, the specific heat , the entropy , and the Helmholtz
free energy of the black hole. The entropy was computed using the Wald
formulation. In addition, the quantum correction to the Wald's entropy is
considered for the string cloud source.
We mainly explore the thermodynamical global and local stability of the
system with vanishing or non-vanishing cosmological constant. The global
thermodynamic phase structure indicates that the Hawking-Page transition is
achieved for this model. Further, we observe that there exist stable black
holes with small radii and that these regions are enlarged when choosing small
values of the string cloud density and of the Gauss-Bonnet parameter. Besides,
the rate of evaporation for these black holes are studied, determining whether
the evaporation time is finite or not. Then, we concentrate on the dynamical
stability of the system, studying the effective potential for s-waves
propagating on the string cloud background.Comment: 13 pages, 5 figures. References adde
Dynamical variables in Gauge-Translational Gravity
Assuming that the natural gauge group of gravity is given by the group of
isometries of a given space, for a maximally symmetric space we derive a model
in which gravity is essentially a gauge theory of translations. Starting from
first principles we verify that a nonlinear realization of the symmetry
provides the general structure of this gauge theory, leading to a simple choice
of dynamical variables of the gravity field corresponding, at first order, to a
diagonal matrix, whereas the non-diagonal elements contribute only to higher
orders.Comment: 15 page
Interacting dark sector with transversal interaction
We investigate the interacting dark sector composed of dark matter, dark
energy, and dark radiation for a spatially flat Friedmann-Robertson-Walker
(FRW) background by introducing a three-dimensional internal space spanned by
the interaction vector and solve the source equation for a linear
transversal interaction. Then, we explore a realistic model with dark matter
coupled to a scalar field plus a decoupled radiation term, analyze the amount
of dark energy in the radiation era and find that our model is consistent with
the recent measurements of cosmic microwave background anisotropy coming from
Planck along with the future constraints achievable by CMBPol experiment.Comment: To appear in the proceedings of "CosmoSur II - Gravitation and
Cosmology in the Southern Cone" (Valparaiso, Chile, 27-31 May 2013
Big brake singularity is accommodated as an exotic quintessence field
We describe a big brake singularity in terms of a modified Chaplygin gas
equation of state p=(\ga_{m}-1)\rho+\al\ga_{m}\rho^{-n}, accommodate this
late-time event as an exotic quintessence model obtained from an
energy-momentum tensor, and focus on the cosmological behavior of the exotic
field, its kinetic energy and the potential energy. At the background level,
the exotic field does not blow up whereas its kinetic energy and potential both
grow without limit near the future singularity. We evaluate the classical
stability of this background solution by examining the scalar perturbations of
the metric along with the inclusion of entropy perturbation in the perturbed
pressure. Within the Newtonian gauge, the gravitational field approaches a
constant near the singularity plus additional regular terms. When the perturbed
exotic field is associated with \al>0 the perturbed pressure and contrast
density both diverge, whereas the perturbed exotic field and the divergence of
the exotic field's velocity go to zero exponentially. When the perturbed exotic
field is associated with \al<0 the contrast density always blows up, but the
perturbed pressure can remain bounded. In addition, the perturbed exotic field
and the divergence of the exotic field's velocity vanish near the big brake
singularity. We also briefly look at the behavior of the intrinsic entropy
perturbation near the singular event.Comment: 11 pages, no figures. Accepted for its publication in PR
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